In the fabrication of semiconductive devices it is often necessary to provide an insulating oxide layer on the substrate. One way in which the oxidation layer is formed is by heating the substrate in an oxidizing atmosphere in a reaction chamber for a time and at a temperature sufficient to provide the desired oxidation layer. One problem associated with high-temperature oxidation of semiconductor substrates is the contamination of the oxide from impurities occurring in the oxidizing ambient including those metal impurities which diffuse from the furnace heating the ambient through the walls of the reaction chamber.
Contamination of the semiconductor substrate due to alkali metals and transition metals can be harmful as known in the prior art. Such harmful effects caused by various alkali metal contaminants and/or transition metal contaminants include degradation of minority carrier lifetimes; increase of junction leakages; and shifts in the flat band voltage.
It has been suggested to provide an HCl protective atmosphere in an outer chamber around the reaction chamber in order to remove metal contaminants. For instance, such has been suggested in U.K. patent application No. 2082384A and J. Electrochm. Soc., Vol. 130, No. 1, pp. 196-199, Schmidt, "Contamination Free High Temperature Treatment of Silicon or Other Materials", January 1983.
However, such technique has not been entirely satisfactory since it has been found that there is a tendency for some contamination due to water or moisture associated with the halogen ambient or reaction products with the halogen.